The invention relates to shock buffers and more particularly to a shock buffer for a control element assembly in a nuclear reactor. More particularly still, part of the buffer mechanism is fixedly positioned in the reactor in spaced relationship with the fuel assemblies of the core.
In a nuclear reactor, control elements are provided for insertion into the core or fuel region of the reactor to control and regulate its reactivity and power level. These control elements contain materials known as poisons which absorb neutrons thereby lowering the local neutron flux. In normal operation, the control elements are withdrawn at least partially from the core region and their position may be controlled to regulate the reactor. In the event of an emergency in which the reactor must be shut down, it is necessary to rapidly insert most or all of these control elements fully into the core. This emergency procedure of inserting control elements is referred to as "scramming".
Control elements which enter the reactor core from above and are withdrawn from the core to a position thereabove can be scrammed by allowing the control elements to fall into the core by the force of gravity. This is normally accomplished by simply disengaging the control elements and drive train from the drive mechanism. It is necessary, however, to provide means for gradually slowing down or decelerating the control elements just prior to their reaching their extreme lower position in the core to prevent damage to the reactor internals or to the control elements.
Various buffering arrangements associated with individual control elements or with a control element assembly including a plurality of control elements have been provided to effect the controlled deceleration of the control elements. One type of buffering means often used is that of a piston and cylinder combination wherein the piston, associated with the dropping control element, forces displacement of a fluid or liquid from the cylinder into which it is entering. The resistance of the fluid to displacement serves to decelerate the dropping control element. One such piston and cylinder type of buffer is shown in U.S. Pat. 3,314,859 issued Apr. 18, 1967 to A. J. Anthony entitled "Nuclear Reactor Fuel Assembly in Control Rod Organization". This buffer is provided by allowing the control element, and particularly the lower end portion thereof, to serve as the piston. With this arrangement, fluid coolant may enter the control element guide tube through an opening positioned several inches above the bottom thereof. When the falling control element is adjacent this opening, it serves to partially block exit of the fluid coolant from the remaining bottom portion of the guide tube and this fluid acts upwardly against the falling control element to effect a gradual deceleration of the control element and apparatus attached thereto.
In another arrangement, the guide tube has a reduced diameter in its lower section to aid in the pressure buildup against the lower end of the falling control element. While providing effective deceleration to the falling control element, this type of buffer has the disadvantage of imposing large compressive axial loading on the control element. These compressive stresses may cause buckling and/or rupturing of the control element due to the significant weight of the control element assembly, most of which is above the buffer region of the control element. Also, because of the large pressure increase in its lower portion, the guide tube may require a reinforcing structure which adversely affects neutron economy in the region. Further too, fluid pressures on the control element, particularly near its lower end during buffering, may require reinforcement of this region of the control element to prevent damage or deformation thereto. Further still, the regions of the control element containing a neutron poison therewithin are subject to some degree of swelling because of irradiation induced volumetric expansion of the commonly used B.sub.4 C neutron poison pellets, thus creating the possibility of the control element jamming in the buffering portion of the guide tube and changes in buffering characteristics. In order to prevent such a situation, this region of the control element replaces the B.sub.4 C poison with silver-indium-cadmium poison which expands less but is more expensive.
Another type of control element buffer using a piston and cylinder arrangement is shown in U.S. Pat. No. 3,518,162 issued June 30, 1970 to H. V. Lichtenberger entitled "Nuclear Reactor Control Element Drive Apparatus". This particular control element buffer is located well above the reactor core region and in fact beyond the reactor pressure vessel and within the housing including the drive mechanism for the control elements. With such an arrangement the piston, to which the control element decelerating forces are applied, is associated with the drive member to which the control element is coupled below. With the piston so positioned, decelerating forces thereupon will act to apply only, or principally, tensile stresses to the control element below, thus avoiding some of the undesirable effects attendant to placing the control element in compression. However, this arrangement also has its deficiencies in that the piston and cylinder combination are located in a region of limited or poor accessibility in the event maintenance or repairs are to be effected. More importantly, if the coupling between the control element drive member and the control element itself should open and disengage the control element, permitting it to drop, the piston associated with the drive member will be ineffective in decelerating the dropped control element.
A control element buffer arrangement which avoids the problem of compressively loading the control element and further, which locates the buffering mechanism in an area of the reactor which is relatively frequently accessible, is depicted and described in U.S. application Ser. No. 234,929 filed Mar. 15, 1972 by Albert Lowery Gaines entitled "Control Element Buffer, now abandoned". This buffer is of the piston and cylinder type and utilizes a fitting, such as an alignment post, associated with and located at the upper end of a fuel assembly as the buffer cylinder. A portion of the control element assembly above the poison region of the control element is sized and shaped to provide the buffer piston. With this arrangement, the piston descending with the control element assembly enters the stationary, upwardly open alignment post, serving as the cylinder and filled with liquid, and decelerates descent of the assembly without compressively loading the control elements.
Another buffer arrangement which avoids the problem of compressively loading the control element and further, may avoid loading the fuel assembly during the buffering operation, is that disclosed in U.S. Pat. No. 3,448,006 issued June 3, 1969 to P. Fortescue et al entitled "Control Rod Drive System". However, this arrangement utilizes a fixedly positioned cylinder which is substantially closed, save for an opening at each end sized only large enough to pass a piston rod continuously therethrough. The cylinder is connected to, and the piston and buffer operated by, a source of pressurized fluid isolated from the coolant fluid of the reactor, thus creating a rather complex buffer.
In most of the above-described buffers, the control element assembly decelerating forces are transmitted to the fuel assembly, causing unnecessary, and possible undesirable stressing thereof. Further, most require the presence of the fuel assembly in order to effect buffering of the rapidly inserted control element assembly, the buffering capability being absent without the fuel assembly. Further still, many of the prior art buffers utilize a coolant flow channel, such as a guide tube, as the buffer cylinder and accordingly, impede or prevent desirable coolant flow when the control element assembly is fully inserted. Still further, most of the buffering arrangements described above have relied upon a stationary, upwardly open cylinder and a downwardly traveling piston to provide the requisite buffering action. In every such instance, the cylinder is oriented such that foreign materials known as crud, circulating with the coolant liquid, may easily accumulate and form deposits therein, thus interfering with proper operation of the piston within the cylinder. While such a problem might be minimized by providing a crud vent opening at or near the bottom of the cylinder, this would require extending the piston-cylinder stroke length to compensate for the liquid lost through the vent opening during buffering. Further too, this measure would only vent crud smaller than a certain particle size.